Radical reaction rates decomposition

K2C03 3 H202 contains hydrogen peroxide of crystallization and the solid phase decomposition involves the production of the free radicals OH and HOi, detected by EPR measurements [661]. a—Time curves were sigmoid and E = 138 kJ mole-1 for reactions in the range 333—348 K. The reaction rate was more rapid in vacuum than in nitrogen, possibly through an effect on rate of escape of product water, and was also determined by particle size. From microscopic observations, it was concluded that centres of decomposition were related to the distribution of dislocations in the reactant particles. 

The rates of radical-forming thermal decomposition of four families of free radical initiators can be predicted from a sum of transition state and reactant state effects. The four families of initiators are trarw-symmetric bisalkyl diazenes,trans-phenyl, alkyl diazenes, peresters and hydrocarbons (carbon-carbon bond homolysis). Transition state effects are calculated by the HMD pi- delocalization energies of the alkyl radicals formed in the reactions. Reactant state effects are estimated from standard steric parameters. For each family of initiators, linear energy relationships have been created for calculating the rates at which members of the family decompose at given temperatures. These numerical relationships should be useful for predicting rates of decomposition for potential new initiators for the free radical polymerization of vinyl monomers under extraordinary conditions. 

A PP sample after ozonization in the presence of UV-irradiation becomes brittle after 8 hrs of exposure, whereas the same effect in ozone is noticeable after 50-60 hours.Degradation of polymer chain occurs as a result of decomposition of peroxy radicals. The oxidation rapidly reaches saturation, suggesting the surface nature of ozone and atomic oxygen against of PP as a consequence of limited diffusion of both oxygen species into the polymer. Ozone reacts with PP mainly on the surface since the reaction rate and the concentration of intermediate peroxy radicals are proportional to the surface area and not the weight of the polymer. It has been found that polyethylene is attacked only to a depth of 5-7 microns (45). 

Radicals are also formed in solution by the decomposition of other radicals, which are not always carbon free radicals, and by removal of hydrogen atoms from solvent molecules. Because radicals are usually uncharged, the rates and equilibria of radical reactions are usually less affected by changes in solvent than are those of polar reactions. If new radicals are being made from the solvent by hydrogen abstraction, and if the new radicals participate in chain reactions, this may not be true of course. But even in cases of non-chain radical reactions in which no radicals actually derived from the solvent take part in a rate-determining step, the indifference of the solvent has perhaps been overemphasized. This will be discussed more fully when radical and polar reactions are compared in Chapter XII. 

Steps (29) and (30) are those involved also in the decomposition of N205. Initial rates of decomposition are apparently higher than at later stages in the reaction because, initially, the free-radical reaction is not inhibited by the fast step (29). When sufficient nitric oxide is present, either initially added or formed by N02 decomposition, the free-radical reaction path is suppressed. Ashmore et al.212 213 found indeed that the value of the second-order rate coefficient of decomposition kd, depends on the [N0]/[N02] ratio in agreement with the relation... 

Vanoppen et al. [88] have reported the gas-phase oxidation of zeolite-ad-sorbed cyclohexane to form cyclohexanone. The reaction rate was observed to increase in the order NaY < BaY < SrY < CaY. This was attributed to a Frei-type thermal oxidation process. The possibility that a free-radical chain process initiated by the intrazeolite formation of a peroxy radical, however, could not be completely excluded. On the other hand, liquid-phase auto-oxidation of cyclohexane, although still exhibiting the same rate effect (i.e., NaY < BaY < SrY < CaY), has been attributed to a homolytic peroxide decomposition mechanism [89]. Evidence for the homolytic peroxide decomposition mechanism was provided in part by the observation that the addition of cyclohexyl hydroperoxide dramatically enhanced the intrazeolite oxidation. In addition, decomposition of cyclohexyl hydroperoxide followed the same reactivity pattern (i.e., NaY < BaY... 

A kinetic study has been carried out in order to elucidate the mechanism by which the cr-complex becomes dehydrogenated to the alkyl heteroaromatic derivative for the alkylation of quinoline by decanoyl peroxide in acetic acid. The decomposition rates in the presence of increasing amounts of quinoline were determined. At low quinoline concentrations the kinetic course is shown in Fig. 1. The first-order rate constants were calculated from the initial slopes of the graphs and refer to reaction with a quinoline molecule still possessing free 2- and 4-positions. At high quinoline concentration a great increase of reaction rate occurs and both the kinetic course and the composition of the products are simplified. The decomposition rate is first order in peroxide and the nonyl radicals are almost completely trapped by quinoline. The proportion of the nonyl radicals which dimerize to octadecane falls rapidly with increase in quinoline concentration. The decomposition rate in nonprotonated quinoline is much lower than that observed in quinoline in acetic acid. 

This generic chain reaction can be sketched similarly to the acetaldehyde decomposition reaction as shown in Figure 10-2. The circular chain propagates itself indefinitely with a rate rp once initiated by rate ri, but it is terminated by rate ri, and in steady state Ti and rt control how fast the cycle runs. The overall reaction rate is controlled by the concentration or the chain-propagating radical Cr because this controls how many molecules are participating in the chain. This is why r and rt are so important in deterniining the overall rate. 

The difference in H2 selectivity between Pt and Rh can be explained by the relative instability of the OH species on Rh surfaces. For the H2-O2-H2O reaction system on both and Rh, the elementary reaction steps have been identified and reaction rate parameters have been determined using laser induced fluorescence (LIF) to monitor the formation of OH radicals during hydrogen oxidation and water decomposition at high surface temperatures. These results have been fit to a model based on the mechanism (22). From these LIF experiments, it has been demonstrated that the formation of OH by reaction 10b is much less favorable on Rh than on Pt. This explains why Rh catalysts give significantly higher H2 selectivities than Pt catalysts in our methane oxidation experiments. 

N02 has also been shown to react with 1,1-dimethyl-hydrazine in air, forming HONO and tetramethyltetra-zine-2, (CH3)2NN=NN(CH3)2 (Tuazon et al., 1983b). The reaction is also proposed to involve abstraction of a hydrogen from the weak N-H bond by N02, forming HONO. The tetramethyltetrazine-2 is hypothesized to be formed by the addition of N02 to the (CH3)2NNH radical, followed by decomposition to (CH3)2N2 + HONO and the self-recombination of the (CH3)2N2 radicals (Tuazon et al., 1982). The apparent overall rate constant for the reaction was 2.3 X 10 17 cm3 molecule-1 s-1 so that the lifetime of 1,1-dimethylhy-drazine at an N02 concentration of 0.1 ppm would be 5 h. Since the lifetimes with respect to 0.1 ppm 03 or 1 X 106 OH radicals cm-3 are 7 min and 6 h, respectively, the reaction of N02 can contribute to the atmospheric reactions of the hydrazine only at low 03 levels. 

Some interesting work on the decomposition kinetics of IC(N02)3 has been reported. In the gas phase this compd decomposes with homogeneous first order kinetics over the temp range of 100-160° (Ref 6). The activation energy obtained (E = 34.4 kcal/mole, log Z = 15.25) suggests that the primary step is the rupture of the C—N bond followed by a radical (nonchain) reaction (Ref 7). Addn of a large excess of NO, one of the decompn products, increased reaction rate 20-30%, addn of a large excess of N02, another decompn product, lowered the reaction rate 10%. Addn of I2, also a decompn product, had no effect on rate (Ref 7)... 

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